Production of explosive azides in molten salt media



United States Patent ()fiFice 3,345,130 PRODUCTION OF EXPLOSIVE AZIDES IN MOLTEN SALT MEDIA Heinrich C Egghart, Hollin Hills, Va., assignor to the United States of America as represented by the Secretary of the Army N Drawing. Filed Sept. 8, 1965,

Ser. No. 485,956 9 Claims. (Cl. 23-101) ABSTRACT OF THE DISCLOSURE The invention described herein may be manufactured and used by or for the Government of the United States for governmental purposes, without the payment to me of any royalty thereon.

This invention relates to an improved process for producing explosive azides, and more particularly, it relates to a process for precipitating lead azide, silver azide, etc. in low-melting salt media.

Many of the azides, especially those of the heavy metals, possess powerful detonating properties. Although explo-, sive azides may be prepared Without undue risk by precipitating solutions of heavy metal salts with an alkali azide, unexplained explosions are known to occur.

The present invention enables explosive azides to be prepared in a manner which minimizes accidental explosions during their formation. The present preparation is especially advantageous because the resulting heavy metal azides, such as lead or silver azide, show increased thermal stability. Unlike previous experiences, larger azide crystals obtained by the present process do not display excessive sensitivity, and crystals of 100200,u. or larger may be precipitated with considerable degree of safety.

It is therefore an object of the present invent to provide an improved process for producing explosive azides wherein the hazards previously associated with azide production have been minimized.

Another object of the invention is to provide a process for the production of lead, silver and other azides which are characterized by a high degree of purity and substantial increase in thermal stability.

A further object of the invention resides in the formation of relatively large lead azide crystals which are thermally more stable and also more resistant to acid action than previous crystals of comparable size.

In accordance with the invention, lead, silver and other azides can be safely prepared by precipitating them in molten salt media. The precipitation may be effected by introducing a heavy metal salt into a solution of alkali azide dissolved in a suitable melt. Alkali nitrate melts dissolve many of the heavy metal salts as well as the alkali azides; the heavy metal azides, however, are for the most part insoluble in these melts. It is now possible, therefore, to precipitate these azides in alkali nitrate melts, and preferably in the low-melting KNO /LiNO and KNO /NaNO mixtures. Melt temperatures preferably of about 150- 250 C. and as high as 300 C. may be employed.

In effect the precipitation of explosive azides may be carried out at any convenient temperature above that of the eutectic mixture of KNO /NaNO /LiNO- which has the lowest melting point at 119 C. Mixtures of KNO and LiNO form suitable melts at 136 C. and above; mix- 3,345,130 Patented Oct. 3, 1967 tures of KNO and NaNO may be used at temperatures of 218 C. and above. An important feature of the present invention is the ionic melt environment in which the precipitation of explosive azides does not experience spontaneous explosions or ignitions due to static electricity. Moreover, the products of the present process exhibit improved stability and may be handled with comparable safety even when larger crystals are produced. Prior art lead size is not in thermodynamic equilibrium because it is prepared approximately 500 C. below the melting point of lead azide. Precipitation in molten salt media permits formation of lead azide at elevated temperatures; therefore, some of the defects which form at room temperature do not occur. Thus, the product precipitated in the melt can be expected to be more uniformly developed.

In the practice of the invention, equivalent amounts of the reactants are brought together in the melt, or a slight excess of the alkali azide may be used. For example, 0.8 gm. of sodium azide, NaN and 2.0 gms. of lead nitrate, Pb(NO are dissolved in separate vessels containing a KNO /NaNO melt at 225 C. The precipitation is carried out by mixing the contents of the two vessels together. The lead azide, PbN which separates in the form of very fine crystals is obtained from the melt by pouring the entire content into water. The alkali nitrate mixture dissolves immediately, and the solution is then filtered through a fritted glass funnel. The residue is then extensively washed with water and rinsed with alcohol or acetone. The latter wash is not essential although it helps to prevent the azide crystals from sticking together and thus become easier to handle. The precipitate is then dried in vacuum over silica gel. Crystallographic analysis has indicated that the precipitate consists of a-lead azide.

Lead azide may also be prepared by dropping lead nitrate powder into a solution of KNO /NaNO melt containing dissolved sodium azide. Lead azide with a purity exceeding 99% has been produced in this manner. If care is taken to dehydrate the nitrate melt, by purging it with dry nitrogen, the purity of the precipitated lead azide will approximate 100%. The precipitated azide is obtained in batches of homogeneous crystal sizes, some batches having an average crystal size of less than 5a and other batches have average crystal sizes as high as 200,41. or more.

Alternately, the lead azide may be separated from the melt by permitting the mixture to cool and solidify at room temperature. The separation is then performed by leaching out the solid salt mixture with Water. The leaching is carried out rapidly by using a magnetic stirrer. The separation may also be performed by initially filtering the lead azide-containing melt through a heated filter. After the lead azide has cooled, it is freed from residual melt by washing it with water, or with a nonaqueous solvent that dissolves alkali nitrates.

The present process is particularly well suited for crystal growth; lead azide crystals up to 10 mm. in length may be produced over an extended period. Crystal growth may be promoted by decreasing the temperature of the melt from 25 0300 C. to a temperature near the melting point of said melt. When long periods of time are involved, as in crystal growth, the present process should be conducted preferably in a dry atmosphere or in a vacuum. Prolonged exposure of azides to humid air would lead to oxy-azide formation.

Lead azide crystals prepared in molten salt media have been found to have a higher thermal stability than the lead azide crystals prepared in the conventional manner in aqueous solution. In the present process the thermal stability of large and small crystals is nearly the same, whereas in the previous preparations the larger crystals were less stable.

When azide crystals grown in alkali nitrate melt were tested in 1% HNO they required 3-4 times longer to Decomposition Temperatures Crystal Sizes (Grown in aqueous solutions), C.

(Grown in melt media), C.

Microscopic crystals (20;; average) Macroscopic crystals (1-5 mm. average).

It will be apparent from the foregoing that the present invention provides a less hazardous process for precipitating explosive azides as well as for growing more stable salts thereof. It may be theorized that the present process develops the crystals at elevated temperatures nearer the melting temperature of the crystals, and therefore the crystal growths are thermodynamically more stable.

It is of course understood that the described embodiments are only exemplary and that various modifications are possible within the scope of the invention as defined in the appended claims.

What is claimed is:

1. The process of precipitating lead azide or silver azide which comprises introducing into a molten alkali nitrate, which is maintained at a temperature in the range of between 119 and 300 C., an alkali azide and a soluble salt in said nitrate, said salt being selected from the group consisting of a lead salt and a silver salt.

2. The process in accordance with claim 1 in which said molten alkali nitrate is selected from the group consisting of a mixture of potassium nitrate and lithium nitrate, a mixture of potassium nitrate and sodium nitrate and a mixture of potassium nitrate, lithium nitrate and sodium nitrate, said nitrate mixtures being maintained at a temperature in the range of between 119 and 250 C.

3. The process in accordance with claim 1 in which said soluble salt is lead nitrate.

4. The process in accordance with claim 1 in which said soluble salt is silver nitrate.

5. The process in accordance with claim 1 in which said alkali azide is sodium azide.

6. The process of precipitating an explosive azide which comprises adding lead nitrate in a solution formed of an alkali azide dissolved in molten alkali nitrate to precipitate said explosive azide, dissolving said solution of alkali nitrate in water to form an aqueous solution and filtering said aqueous solution to extract said explosive azide.

7. The process in accordance with claim 6 in which said solution is at a temperature in the range of about to 250 C. and said alkali nitrate consists of a mixture of potassium nitrate and sodium nitrate.

8. The process in accordance with claim 6 which includes the step of drying said explosive azide.

9. The process in accordance with claim 6 wherein said process is conducted in a dry atmosphere.

Ser. No. 292,742, Beck et al. (A.P.C.), published July 1943.

OSCAR R. VERTIZ, Primary Examiner.

A. J. GREIF, Asisistant Examiner. 

1. THE PROCESS OF PRECIPITATING LEAD AZIDE OR SILVER AZIDE WHICH COMPRISES INTRODUCING INTO A MOLTEN ALKALI NITRATE, WHICH IS MAINTAINED AT A TEMPERATURE IN THE RANGE OF BETWEEN
 11. AND 300*C., AN ALKALI AZIDE AND A SOLUBLE SALT IN SAID NITRATE, SAID SALT BEING SELECTED FROM THE GROUP CONSISTING OF A LEAD SALT AND A SILVER SALT. 